Hydrogen will become a major energy carrier in a future energy regime based on renewable resources. Water electrolysis is the most practical way to produce hydrogen using renewable resources. Investment and production costs of electrolysers define the total economy of the system and will determine whether this is to become a feasible process for hydrogen production. The production cost of hydrogen by water electrolysis is, to a large extent, affected by the electric power consumption, which can be about 70% of the total production costs of hydrogen.
Two different types of water electrolysers are commonly used in the state of the art: Alkaline electrolysers and PEM water electrolysers. Water electrolysers using a polymer electrolyte membrane (“PEM”) along with precious metal catalysts are able to operate at considerably higher current densities and at lower specific energy consumption compared to conventional alkaline electrolysers giving the advantage of higher utilisation of the equipment and reduced production costs. In the best PEM electrolysers, a cell voltage of 1.67 V at 3 A/cm2 has been obtained. This cell voltage is comparable to that of a modern alkaline electrolyser which typically is operating at 0.2 A/cm2. This means that the alkaline electrolyser needs 15 times larger active area to produce the same amount of hydrogen at the same electrical power consumption compared to a PEM electrolyser system.
The present invention is therefore directed to improvements of catalysts for PEM water electrolysers.
In principle, PEM water electrolysers are built up similar to a PEM fuel cell, however, they are working in a different manner. During PEM fuel cell operation, oxygen reduction takes place at the cathode and hydrogen oxidation occurs at the anode of the fuel cell. In summary, water and electrical current is produced. In a PEM water electrolyser, the current flow and the electrodes are reversed and water decomposition takes place. Oxygen evolution occurs at the anode (abbreviated “OER”=oxygen evolution reaction) and reduction of protons (H+), which travel through the polymer electrolyte membrane, takes place at the cathode (abbreviated “HER”=hydrogen evolution reaction). As a result, water is decomposed into hydrogen and oxygen by means of current. The reactions can be summarized in the following equations:2 H2O=>O2+4 H++4e− (OER)4 H+4e−=>2 H2 (HER)
The PEM water electrolyser generally comprises a polymer electrolyte membrane (for example Nafion® by DuPont), which is sandwiched between a pair of electrode layers and a pair of porous current collectors (or gas diffusion layers) mounted respectively on both sides of the electrode layers.
In PEM fuel cell electrodes, platinum on carbon catalysts are used for both, the anode electrocatalyst (for hydrogen oxidation) and the cathode electrocatalyst (for oxygen reduction). In the PEM electrolyser, carbon based materials such as Pt/carbon catalysts and carbon-fiber based gas diffusion layers (GDLs) cannot be used at the anode side because of corrosion of carbon by the oxygen evolved during water electrolysis.
For the manufacture of a membrane-electrode-assembly for a PEM electrolyser, catalyst inks comprising catalyst powders, solvents and optionally polymer electrolyte (i.e. “ionomer”) material is prepared and applied either directly to the membrane or to the gas diffusion layer and then contacted with the membrane. The manufacture of this assembly is similar to the manufacture of membrane-electrode-assemblies (MEAs) for PEM fuel cells, which is broadly described in the literature (see for example U.S. Pat. No. 5,861,222, U.S. Pat. No. 6,309,772 and U.S. Pat. No. 6,500,217).
Among all precious metals, platinum is the most active catalyst for the hydrogen evolution reaction (HER) at the cathode and can be applied at moderate loading. Iridium and iridium oxide is well known for its unique electrocatalytic properties in respect to chlorine and oxygen evolution processes (ref to DEGUSSA-Edelmetalltaschenbuch, Chapter 8.3.3, Huethig-Verlag, Heidelberg/Germany, 1995). Thus, iridium is the preferred material for the oxygen evolution reaction (OER) at the anode side, either in the form of pure metal or as oxide. However, for certain purposes, other precious metal oxides (preferably oxides of ruthenium or platinum) may be added.
In PEM water electrolysers, the precious metal catalyst loading on the anode and on the cathode is still relatively high, 3-5 mg p.m./cm2 or more. Therefore there is a need for the development of improved catalysts with lower oxygen overvoltage and longer service life, which allows to reduce the catalyst loading of the electrolysers.